Turn style lints to warn by default

[brson]

Our stylistic lints are set to allow currently, which makes them very ineffective at promoting a consistent style across the Rust ecosystem. We should turn them on and force users to opt out of the community style.

[lambda-fairy]

Looks like #11457 has fixed the issue. Should we close this?

[huonw]

#11457 hasn't landed yet.

[mrshu]

I was trying to set NonUppercaseStatics to warn and fix errors that might occur but I found out that many things depend on each other, like io/mod or io/process. What do you think should the right approach be? Just putting #[allow] there?